Reversible Diels–Alder Addition to Fullerenes A Study of Dimethylanthracene with H2@C60 2022 Mahboob Subhani.pdf

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nanomaterials
Article
ReversibleDiels–AlderAdditiontoFullerenes:AStudyof
******@C60
MahboobSubhani1,†,JinrongZhou1,†,YuguangSui1,HuijingZou2,MichaelFrunzi3,JamesCross3,
MartinSaunders3,CijunShuai4,*,WenjieLiang1,*andHaiXu1,5,*
1CollegeofChemistryandChemicalEngineering,CentralSouthUniversity,SouthLushanRoad,
Changsha410083,China;******@(.);*************@(.);
sui_******@(.)
2DepartmentofBiology,CollegeofArtsandScience,NewYorkUniversity,NewYork,NY10012,USA;
******@
3DepartmentofChemistry,YaleUniversity,NewHaven,CT06520,USA;micheal.******@(.);
james.******@(.);******@(.)
4CollegeofMechanicalandElectricalEngineering,CentralSouthUniversity,SouthLushanRoad,
Changsha410083,China
5ShenzhenResearchInstitute,CentralSouthUniversity,High-TechIndustrialPark,YuehaiStreet,
Shenzhen518057,China
*Correspondence:******@(.);liang_******@(.);******@(.)
†Theseauthorscontributedequallytothiswork.
Abstract:Thestudyofisolatedatomsormoleculesinsideafullerenecavityprovidesaunique

–Alderadditionreactionof
9,10-dimethylanthracene(DMA)******@C60while1HNMRspectroscopyisutilizedtocharacterize
theDiels–,aseries
Citation:Subhani,M.;Zhou,J.;Sui,,precise,andstraightforward.
Y.;Zou,H.;Frunzi,M.;Cross,J.;Moreover,inthispaper,******@C60anditsisomersaredescribedfortheflrsttime.
Saunders,M.;Shuai,C.;Liang,W.;
Xu,–AlderKeywords:1HNMRspectroscopy;Diels–Alderreaction;9,10-dimethylanthracene;fullerene;******@C60
AdditiontoFullerenes:AStudyof
******@C60.
Nanomaterials2022,12,://
/
AcademicEditor:
classofformhasbecomeaparticularlystrikinganddynamicresearcharea[1,2].In
Received:30March2022
Accepted:29April2022the1990s,theformationofanoriflceonfullereneC60attractedimmenseattentionfortar-
Published:13May2022getingendohedralfullerenesinorganicsyntheses,suchasmolecularcontainers[3–8],and
,endofullerene
Publisher’sNote:MDPIstaysneutralcontainssmallatomsormolecules,includinghelium,hydrogen,andwater,aswellasen-
withregardtojurisdictionalclaimsindohedralmetal-fullerenes,suchasLi,Th,andScN[9–14].Insertingsmallmoleculesor
3
publishedmapsandinstitutionalaffll-atomsintoabigclosedfullerenecageisanintriguingyetdemandingresearchareafor
,theencasedmoleculeperformslikeaconflnedquantumrotor,witha
sophisticatedenergylevelstructure[15]thatcouldbeinvestigatedvianeutronscattering,
infrared(IR),aswellasnuclearmagneticresonance(NMR)spectroscopy[15–21].The
Copyright:©2022bytheauthors.******@C60,******@C60,******@C60make
LicenseeMDPI,Basel,
Thisarticleisanopenaccessarticlewitheaseandprecision[18,22–25].Fullerene’sexteriorchemicalfunctionalitiesareheavily
distributedunderthetermsandinfluencedbytheirbondreactivity.
conditionsoftheCreativeCommonsTheDiels–Alderadditionreactionisavaluableresearchareainfullerenechemistry.
Attribution(CCBY)license(https://Tofunctionalizefullerenes,theDiels–Alderreactionisused[26].Whensmallmolecules
,theynotonlyenhancethestabilitybutalso
/).increasetheDiels–Aldercycloadditionreactivityofthefullerene[27–29].Ontheonehand,
Nanomaterials2022,12,:///nano12101667:.
Nanomaterials2022,12,xFORPEERREVIEW2of12
Nanomaterials2022,12,1667oratomsareencapsulatedinthefullerene,theynotonlyenhancethestabilitybutalso2of11
increasetheDiels–Aldercycloadditionreactivityofthefullerene[27–29].Ontheone
hand,ithasbeendemonstratedthattheDiels–Alderreactionof9,10-dimethylanthracene
ithasbeendemonstratedthattheDiels–Alderreactionof9,10-
C[30].Ontheotherhand,-70[30].
Ontheotherhand,
-
thereactiontomanipulatethepositionofseveralmalonateadditionsbyaddingremovableingremovabletemplatestoC60(seeFigure1)[30–32].Inanearlypublication,wesuc-
templatestoCcessfullyemployedheliumspectroscopytoexplainthereactionsofC60(seeFigure1)[30–32].Inanearlypublication,wesuccessfullyemployed60andC70with
heliumspectroscopytoexplainthereactionsofCDMAandstudiedtheequilibriaofthereaction[33].Whenheliumisinsertedinsidean60andC70withDMAandstudiedthe
equilibriaofthereaction[endohedralfullerenecage,anditsNMRchemicalshiftismatchedwiththechemicalshift33].Whenheliumisinsertedinsideanendohedralfullerene
cage,anditsNMRchemicalshiftismatchedwiththechemicalshiftofof3Heoutside,themagneticfieldshieldedbythefullerenecanbemeasured3Heoutside,the
magneticfleldshieldedbythefullerenecanbemeasureddirectly[directly[27,34–41].Thisiscausedbydiamagnetismandislinkedtotheringcurrentsin27,34–41].Thisiscaused
,******@Cn,havepreviouslybeen
Higherfullerenes,******@CdemonstratedtohaveheliumNMRchemicaln,havepreviouslybeendemonstratedtohaveheliumNMRshiftsthatfallbetweentwoextremes,
chemicalshiftsthatfallbetweentwoextremes,namelylow-fleldCnamelylow-fieldC60andhigh-fieldC70,indicatingthattheyhavean“intermediate”ar-60andhigh-fleldC70,
indicatingthattheyhavean“intermediate”aromaticnature[omaticnature[37,39].The3HeNMRisvitalforfullerenechemistry[42,43].Still,thereare37,39].The3HeNMRisvital
forfullerenechemistry[somelimitationstoheliumNMR,.,th42,43].Still,therearesomelimitationstoheliumNMR,.,theemeasurementatlowtemperaturedemands
measurementatlowtemperaturedemandshighsensitivity,lownoise,,lownoise,,
EveniftheNMRsignalitselfisvisible,,3Hecanbe
,3Hecanbestudiedinaspeciallydesignedandconstructed
-
,orKCN,whichishighlytoxic,astassiumcyanide,orKCN,whichishighlytoxic,aswellasspecial3Heisotopes,whichare
wellasspecial3Heisotopes,,simple,
,simple,andeasytooperate,thespectrafor
andeasytooperate,.
-adductofDMAandCGaussviewofmono-adductofDMAandC60..
60
Inthecontinuationofourpreviouswork[Inthecontinuationofourpreviouswork[44],weusedtheT-JumpMethodtothe44],weusedtheT-JumpMethodtothe
Diels–Alderadditionof9,10-dimethylanthracenetoCDiels–Alderadditionof9,10-
******@******@******@******@,,******@C602toin-***@C60
toinvestigatetheDiels–Alderadditionreactionof9,10-–Alderadditionreactionof9,10-dimethylanthraceneandfullerene.
Through1HNMRcharacterizationandanalysis,wefound1mono-,6bis-,11tris-,and10
tetrakis-adducts.
:.
Nanomaterials2022,12,16673of11

******@C60waspreparedbyfollowingthepreviouslyde-
scribedmethod[45,46].NMRtubeswereusedtoprepareallofthe1HNMRsamples.
Atotalof3.******@,1mLof
mixedsolventCS2:CDCl3=4:******@C60completely,andthena1H
,
()(
ofDMA(1equiv)weremixedin1mLofCS2:

60
NMR~30minafterthemixing)wasallowedtostandovernight,andthenthecorrespond-
,aweightedamountof
-
ratedsolution,
(1equiv),(),(2equiv),(),
(4equiv),(6equiv),(10equiv),(15equiv),
(20equiv)untilthesaturationsolutionisreached.
ProtonNMRSpectroscopyA500MHzBrukerAvancespectrometerobtained1H
-gatingpulsesequencewasadoptedtoremoveasubstantialsignal
fromtheringprotonsonthenon-
diminishtheinstrument’sautomaticgainsetting,
totalof256scanswereacquiredforthecompetitivereactionwitha3-secondacquisition
time,a2-secondrecyclingdelay,anda5Hzlinebroadening.

WhentheDMAconcentrationisincreasedatroomtemperature,theDiels–Alder
additionofDMAtoC60becomesreversibleandyieldsmono-,bis-,tris-,andtetrakis-
adducts,
istaken.******@C60werepreparedin4:1CS2:CDCl3.
Aftertheestablishmentofequilibrium,
60
reactswithDMA,anewpeakintheDMAproductpeakappearsat=,,and
near=–,althoughthespectralpeaksarecomplicated

andDMAlowersC60’ssymmetry,andtheadditionproductisanisomerwiththesame
symmetricalfeaturesasC,
60
result,correspondingtothenewspectralpeaksonebyoneusing1HNMRisdifflcult,as
illustratedinFigure2.
Inaddition,******@C60withDMA,theadditionproduct’s1HNMRpeak
appearedinthehighfleldat=,whichiseasiertoexaminesincethecorrespond-

withmoreDMAmoleculescorrespondtothefaster-increasingpeakifthisratiovarieswith
,thereisonlyonemono-adduct
,itspresencecanbefound
byanalyzingthe1HNMRspectra(
C60isexcessiveatthistime,theDiels–AlderadditionreactionbetweenDMAandC60is
dominatedbymono-adducts,andmono-,we
speculatethatthepeakat=-
Diels–AlderadditionreactionwithC60cannotgeneratethetris-andtetrakis-adductsdueto
thesmallamountofDMA,ortheconcentrationofthetris-andtetrakis-adductsistoolow
,wespeculatethatthepeaksat==
areduetoasmallnumberofbis-
(asshowninFiguresS2andS3SupplementaryMaterials),theanalysis
ofthe1HNMRspectrashowedthatatthistime,******@Cwasrelatively
260
low,thatis,******@C60continuedtoreactwiththenewlyaddedDMA,and
theintensityofthespectralpeakat=,thatis,th